Interpretive Summary: Some crops have been genetically altered to be more resistant to herbicides, chemicals that are used to control weeds. However, many weeds in agriculture are closely related to the crop. As a result there is concern that genes associated with genetically altered crops can outcross or "jump" to their weedy relatives with negative consequences for weed management. In this study we examined whether recent and projected increases in atmospheric carbon dioxide could increase the flow of genes (like those that cause herbicide resistance) between wild and genetically altered rice populations. We were able to show that rising CO2 increased the flow of genes from wild, weedy rice to the herbicide resistant, genetically altered, rice population. The increase in pollen outcrossing and gene transfer was associated with a greater relative response of the wild rice compared to the cultivated rice population. When cultivated rice received these genes, it became an unwanted or weedy rice plant, one that has resistance to herbicides. The observed changes reported here suggest that differential responses to rising atmospheric CO2 could result in enhanced flow of novel genes between rice populations and more difficult chemical management of wild rice in rice production systems.

Technical Abstract:
Although recent and projected increases in atmospheric carbon dioxide can alter plant phenological development, these changes have not been quantified in terms of floral outcrossing rates or gene transfer. Could differential phenological development in response to rising CO2 between genetically modified crops and wild, weedy relatives increase the spread of novel genes, potentially altering evolutionary fitness? Here we show that increasing CO2 from an early 20th century concentration (300 µmol mol-1) to current (400 µmol mol-1) and projected (600 µmol mol-1) values, increased the flow of genes from wild, weedy rice to the genetically altered, herbicide resistant, cultivated population, with outcrossing increasing from 0.22% to 0.71% over the range of CO2 values. The increase in outcrossing and gene transfer was associated with differential increases in plant height, as well as greater tiller and panicle production in the wild, relative to the cultivated population in response to increasing CO2. In addition, increasing CO2 also resulted in a greater synchronicity in flowering times between the two populations. The observed changes reported here resulted in a subsequent increase in rice dedomestication and a greater number of weedy, herbicide-resistant hybrid progeny. Overall, these data suggest that differential phenological responses to rising atmospheric CO2 could result in enhanced flow of novel genes and greater success of feral plant species in agroecosystems.